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Question

Does OpenMP natively support reduction of a variable that represents an array?

This would work something like the following...

float* a = (float*) calloc(4*sizeof(float));
omp_set_num_threads(13);
#pragma omp parallel reduction(+:a)
for(i=0;i<4;i++){
   a[i] += 1;  // Thread-local copy of a incremented by something interesting
}
// a now contains [13 13 13 13]

Ideally, there would be something similar for an omp parallel for, and if you have a large enough number of threads for it to make sense, the accumulation would happen via binary tree.

Answer

OpenMP can perform this operation as of OpenMP 4.5 and GCC 6.3 (and possibly lower) supports it. An example program looks as follows:

#include <vector>
#include <iostream>
int main(){
  std::vector<int> vec;
  #pragma omp declare reduction (merge : std::vector<int> : omp_out.insert(omp_out.end(), omp_in.begin(), omp_in.end()))
  
  #pragma omp parallel for default(none) schedule(static) reduction(merge: vec)
  for(int i=0;i<100;i++)
    vec.push_back(i);
  for(const auto x: vec)
    std::cout<<x<<"\n";
  return 0;
}

Note that omp_out and omp_in are special variables and that the type of the declare reduction must match the vector you are planning to reduce on.